SN Instructed that Doxycycline is known as a tetracycline antibiotic. It works by stopping the growth of bacteria. This antibiotic treats only bacterial infections. Possible side effects: Stomach upset, diarrhea, nausea, or vomiting may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor right away if you have any serious side effects, including: sunburn (sun sensitivity), painful/difficult swallowing, change in the amount of urine.